Same is happening to me. Join Game button is deactivated and tells me that it is only available for the party leader… and I am not in any party. Tried also restarting, changing pet, char, banner, seasonal, non seasonal… but nothing worked.
I opened a ticket and they replied that this was a known bug and they are currently trying to fix it.
So no D3 today it seems.
Let’s hope this gets fixed quickly.